The fastener assembly of the invention is found especially useful in the field of gas turbine engine design and manufacture. In a traditional gas turbine engine design a rotor comprises a disc having a central cob or hub, a rim and a web between them, the blades are spaced apart around the rim. Originally the rotors were mounted on a drive shaft individually. Such arrangements tend to be heavy, difficult to balance, relatively easy but time-consuming to assemble and disassemble. Improved rotor designs in which several rotors are bolted or welded together into unitary drum like structures are easier to balance. The drum is then mounted on the drive shaft as a single unit, rather than as a plurality of individual rotors, using a fastener assembly such as a securing nut engaged with a threaded portion on the shaft butting against a shoulder on one of the rotor discs.
Essentially the design of the individual rotor stages retains a central hub, web and rim. Access requirements for mounting rotor drums on a shaft during initial assembly, repair and maintenance impose a minimum bore diameter on the inner circumference of the hub which translates into a significant weight penalty in order adequately to contain stresses in the rotor design. An alternative to increased bore sizes is to use a trapped nut, but this has a detrimental effect on engine assembly and maintenance, especially as self-locking nuts need to be replaced periodically in order to avoid loss of locking torque. As a result the rotor drum has to be split and then rejoined to allow replacement of the trapped nut leading to inevitable drawbacks as previously mentioned. The present invention is intended to address these problems by providing an alternative route for insertion and retrieval of the nut otherwise than through the central bore of the discs.
The invention is not limited to use in conjunction with gas turbine engines and may be utilised in other fastener assemblies that use a trapped nut.